1. Field of the Invention
The invention relates to a press for pressing pulverulent materials, in particular metal powder, having an eccentric crank drive, which has at least one connecting rod and a crankshaft, for driving an upper punch unit. The invention further relates to a method for operating this press.
2. Discussion of the Prior Art
For many years, mechanical powder presses have been used in the technology of pressing metal and metal ceramic powders in order to produce powder compacts. These mechanical presses, which are usually designed as eccentric presses or toggle presses, are distinguished by a high working speed combined with a sinusoidal punch movement with a very progressive compressive force profile during the working cycle. To produce particularly complicated shaped parts, it is preferable to use powder presses whose press tools are moved by hydraulic piston/cylinder systems. In conjunction with corresponding electronic control units, the individual press tools can be optimally controlled, with regard to compressive force and pressing travel, in such a way that the compacts formed, despite their complicated shape, are distinguished by a density inside the shaped body volume which is as far as possible constant. Compared to mechanical presses, however, hydraulic presses generally present a lower working speed, i.e. longer cycle times, and consume considerably greater amounts of energy.
German reference DE 41 14 880 A1, which represents the generic prior art, discloses a press for pressing pulverulent materials which is designed as a mechanical eccentric press having an electric drive motor for moving the upper punch of the press. The crankshaft of the eccentric drive for the upper punch its connected in a rotationally fixed manner to a gearwheel which is moved by a worm drive which, for its part, is rotated by an electric motor. The direction of rotation of the electric motor and the crankshaft does not change during operation. A hydraulic piston/cylinder system is provided in order to move the die. The particular feature of this known press is that it has a coding switch which scans the working position of the upper punch and feeds a corresponding signal to the electronic control unit of this press. Furthermore, there is a frequency converter which acts on the electric drive motor and receives setting signals from the electronic control unit, so that the drive movement can be controlled. The upper punch is mounted in a pressure-measuring cylinder and can be displaced in the pressing direction. The hydraulic displacement of the upper punch is guided by the electronic press control unit. The intention of this combination of a mechanically driven eccentric press with additional hydraulic drives for press tools is that it should be possible to produce large numbers of powder compacts which are also of very good quality in terms of their shape, while ensuring constant dimensions and density of the compacts.
The object of the present invention is to refine a press of the generic type in such a way that the sinusoidal movement and progressive force profile which is known from mechanical presses and is advantageous for the compression of the powder is combined with the advantages which are brought about by a relatively simple hydraulic driving technology in terms of a high level of press flexibility and a pressing profile which is close to ideal together with a high level of reproducibility of speed and position of the press tools. The energy consumption by this press is to be low in relation to the drive forces which it is able to generate. The pressing parameters are to be easily adjustable in order to optimize the movement sequence and the power requirements. Furthermore, it is intended to provide a method for operating this press.
To drive its upper punch unit, the press according to the invention has an eccentric crank drive which comprises at least one connecting rod (usually arranged in pairs), which at one end is connected to the upper punch unit and at the other end is eccentrically connected to a crankshaft. The connection to the crankshaft may, for example, be produced by means of an eccentric disc. A gearwheel is connected to the crankshaft in a rotationally fixed manner. This gearwheel can be rotated by at least one, preferably by two, drive worms which are expediently situated diametrically opposite one another with respect to the crankshaft and for their part are driven by at least one motor, preferably are each driven by a separate motor. The movement sequences of this press are guided by an electronic control unit. The essential distinguishing feature of the invention is that this electronic control unit is designed for reversing operation of the crankshaft. The crankshaft is preferably rotated through an angular range of less than 180xc2x0. In accordance with the reversing rotation of the gearwheel, the force transmission through the connecting rod causes the upper punch unit to move up and down, i.e. to and from between the pressing position and the filling/ejection position. Unlike in conventional mechanical presses with an eccentric crank drive, in the case of the press according to the invention the crankshaft therefore does not execute complete revolutions.
Due to the particularly high torque density in relation to the volume and the relatively low flywheel effect GD2 of hydraulic motors, which allow highly dynamic driving, hydraulic motors are preferred to the use of electric drive motors. The arrangement of two worm drives each with a separate drive motor allows the worm drive to generate torques on the crankshaft which are twice as high, with a relatively small volume, due to the force transmission ratio, without the tooth loads on the gearwheel or the worm drives increasing.
It is particularly expedient if the control unit is designed so that the preferably two hydraulic motors of the press can optionally be connected in parallel and in series in terms of the way in which they are connected into the circuit of the hydraulic medium. In the case of a parallel circuit, with two hydraulic motors, half the mass flow passes through each motor, while in the case of a series circuit the entire mass flow passes through each of the two motors. This offers the possibility, without changing the hydraulic unit, of setting a working speed which is standard or twice as high. The latter is very particularly advantageous in particular for pressing relatively small parts of low height.
Furthermore, it is advantageous if the press includes a die which can be displaced in a controlled manner by hydraulic cylinders in continuous-path control, as is fundamentally known for hydraulic presses. Furthermore, the press may comprise a hydraulically actuable tool adapter. For these cases, it is expedient to provide a central electric motor which drives a hydraulic pump for the upper punch unit and a further hydraulic pump for the hydraulic cylinders of the die and/or the hydraulically actuable tool adapter.
To determine the particular position of the upper punch unit, it is recommended to use electronic measuring systems for indirect or preferably direct determination. By way of example, it is possible to provide an electronic displacement-measuring system for recording the current position of the upper ram of the press, which holds the upper punch unit, or alternatively an electronic rotation angle transmitter for recording the current angular position of the crankshaft.
The particular advantage of the press according to the invention, the movements of whose press tool parts are guided by the electronic control unit, is that it is possible to directly influence the driving of the eccentric crankshaft, preferably by means of the flow of hydraulic medium, which is very easy to influence hydraulically using simple means with regard to its volumetric flow rate and pressure. Therefore, both the speed and the torque at the eccentric crank drive can be influenced very easily and accurately by hydraulic means. A further advantage is that the eccentric crankshaft results in a considerable transmission ratio with regard to the compressive force which can be generated by the press. Naturally, the compressive force required is greatest in the region of the top dead center of the upper punch unit. However, it is in this very position of the press that the transmission ratio between driving force and compressive force is also at its greatest. This means that the driving power required for the press drive can be selected to be significantly lower compared to a hydraulic press which has the same maximum compressive force. Consequently, the total energy consumption during a pressing cycle is also significantly lower.
The press according to the invention allows cycle times which are shorter than those of a continuous mechanical eccentric crank press which is driven in the usual way by an electric motor. This is possible if the press control unit is set in such a way that the stroke is terminated and then reversed in each case significantly before the top dead center of the eccentric crank drive is reached in a conventional mechanical press, this movement always has to be fully completed.
The cycle time of a conventional mechanical press is determined to a significant extent by the procedures required during removal of the compact. These include in particular the need to maintain a loading force while the die is being pulled away, this force being applied by a hydraulic piston/cylinder system accommodated in the upper punch drive unit. In continuous operation, this piston/cylinder system has to carry out an extension movement corresponding to the return movement of the upper punch drive unit in order to maintain the loading force and, after the die has been pulled away, has to be retracted into the starting position as quickly as possible. This requires either a particularly high-performance (expensive) hydraulic system or an adjustment of the basic speed (rotational speed) of the press to match the time required for movement of the piston/cylinder system. In the press according to the invention, the speed of the upper punch drive unit can be considerably reduced or even temporarily held at zero in the region of the top dead center without any problems, until the compact has been released. As a result, a hydraulic outlay on the cylinder movements required for the loading force can be kept very low. After demolding, the upper punch drive unit can be returned to its starting position at the maximum possible speed.
Advantageous operation of the press according to the invention is also produced if the stroke in the region of the bottom dead center of the upper punch unit is set in such a way that the bottom dead center is overrun slightly. The press is therefore operated in the region of a crank angle which is slightly above 180xc2x0 (absolute angle). After the end point has been reached, the fundamentally reversing operation of the press means that the dead center is inevitably crossed again at 180xc2x0. This means that double pressing with the maximum compressive force at the bottom dead center is carried out for each working cycle in an extremely simple way. For certain pressed parts, this is particularly advantageous.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.